The bridge deck broke at the intersection of two of the diagonal truss members. This is called a truss "panel point," and it's where stress concentrates in a truss. The roof broke at the upper panel point.
Video report on the phone call:
https://www.youtube.com/watch?v=FNrAMYxp8CEBeing an olde pharte who has lived through several infamous collapses and read the reports on the causes, I'm baffled at the hubris that led them to open the road to traffic under a bridge that didn't have the structural cable system in place.
In April of 1987, a 16-story building was under construction in Bridgeport, Connecticut. The structural system was called "lift slab." In this system, the concrete floor slabs are all poured on the ground, one on top of another like a stack of pancakes. Once they're cured, they are hoisted up and clipped in place to the vertical columns. At L'Ambiance Plaza, something went wrong and one of the slabs came crashing down, taking down the entire structure and killing a number of workers who were under it. In the aftermath, Connecticut enacted a moratorium on lift slab construction (which may still be in effect), and the industry adopted rules that only essential workers are allowed under the slabs while they are being hoisted and secured.
This bridge is kind of the same idea. Instead of "lift slab," it's a "lift bridge." The idea of building bridges off to the side and then dropping them into place is not new, but in this case the bridge they dropped into place was NOT the complete structure. The completed structure would have included the central pylon (which hadn't even been built yet), the other span (across the canal), and the cables connecting and supporting the two spans. Obviously, someone thought the truss was structurally sufficient without the cables. That calculation was obviously incorrect. (Either that, or there was a massive flaw in the construction. That's possible, but probably unlikely.)
I just can't believe they opened the road to traffic with the structure not fully in place.